1. Field of the Invention
The present invention relates to lift axle suspension systems and, more particularly, to a lift axle suspension system having an air spring lift member.
2. Description of the Related Art
A variety of lift axle suspension systems are known in the art. Many heavy-duty trucks and similar vehicles utilize one or more liftable auxiliary axles also referred to as lift axles. When the truck is carrying a heavy load, the lift axle will be lowered so that the wheels mounted on the lift axle can engage the road surface and the lift axle can bear a portion of the vehicle load. The axle is lifted out of service, with its wheels disengaged from the road surface, when the truck is empty or carrying only a light load and the additional load carrying capacity provided by the lift axle assembly is not required.
Such lift axles often employ two identical parallelogram linkages located on opposite sides of the vehicle. Each of the parallelogram linkages includes an upper support arm and a lower support arm. Typically, the upper and lower support arms are pivotally mounted on a support hanger that extends downwardly from a longitudinal rail member of the truck chassis on one end and pivotally connected to the lift axle on their opposite ends. A support spring, typically an air spring, is provided between the axle assembly and the chassis to transfer loads between the truck chassis and the axle assembly when the axle is lowered and the attached wheels engage the road surface. A second air spring is conventionally used to provide a lift spring for the axle assembly. Such lift axles may also be steerable axles and include steering mechanisms.
When it is desired to lower the lift axle, the support air spring positioned between the axle assembly and chassis is energized or inflated to extend the support spring and the lift air spring is simultaneously de-energized or deflated to retract the lift spring. To raise the lift axle, the support air spring is de-energized or deflated to retract the support spring and the lift air spring member is energized or inflated to extend the lift spring. Known lift axle suspensions have employed lift spring members in a number of different positions for lifting the axle assembly. While such known assemblies are effective, an improved lift axle suspension system that is rugged, reliable and that can be cost effectively manufactured is desirable.